The experiments we are conducting are designed to gain insight into the structural organization of the cardiac actin molecule, through characterization of its antigenic properties and antigenic determinant relationships to other surface binding properties of the molecule. We have demonstrated, by radioimmunoassay, that actins, although a highly conserved family of proteins, are antigenically distinguishable from one another. We are now using RIA and immunofluorescence to determine if cardiac actin has species-specific antigenic determinants, the relationship between conformational state of cardiac actin immunogen and the actin type specificities of resulting antibodies, to localization of antigenic determinants to specific sequence regions of the cardiac actin molecule, and the functional relationship between antigenic determinants and the profilin and DNAase binding sites on the cardiac actin molecule.